Nanometric fine particles are being studied in electronics, chemistry, machine metal industry, and other fields. Of particular interest for these industries are clusters representing aggregates of several thousand molecules, and superfine clusters comprising anywhere from several to several hundred atoms. The issue of controlling the synthesis, surface improvement, and fine structure of such nanometric fine particles occupies an important position in the aforementioned fields. There are, however, no reports on established methods for synthesizing hollow spherical particles of nanometric size capable of being used for microcapsules and the like.
Naturally occurring aluminosilicates shaped as hollow spheres are known as weathered volcanic glass, but precision techniques such as high-efficiency, high-purity separation/extraction have not yet been established for obtaining such fine particles, and additional research is being conducted.
An attempt has also been made to artificially synthesize amorphous aluminosilicate clusters shaped as such hollow spheres (Wada, S., Nendo Kagaku, Vol. 25, No. 2, pp. 53-60, 1985), but successful results can only be obtained when the starting material is a solution diluted to about 2-4 mmol, which is disadvantageous because the resulting yield is very low, the purity of the product is also low due to the formation of by-products, and the like. A synthesis method has also been proposed for raising the concentration of nanotubular silicates called imogolite to 60 mmol by making use of organosilicon and aluminum compounds (GB Patent 1,574,954 and U.S. Pat. No. 4,252,779) , but there are no examples in which hollow spherical particles are synthesized in high concentrations.
The concentration of starting materials and the acidity or basicity of solutions must be taken into account in order to synthesize hollow spherical silicate clusters having such compositions or structures.
According to a conventional technique, a sodium hydroxide solution is added to a mixture of a monomer silicic acid compound solution (2 mmol or lower) and an aluminum compound solution (0.5-4 mmol) such that the NaOH/Al ratio is 3.0 in order to control the rate of polymerization, and the system is kept at 100.degree. C. for about 5 days, yielding a hollow spherical silicate.
The aforementioned synthesis techniques are incapable of yielding such materials with high efficiency because of the conventional notion that amorphous silica or a hydroxide (boehmite, gypsite, or the like) precipitates first unless the starting materials are gradually mixed in a low concentration under near-equilibrium conditions, and the pH is slowly neutralized with an alkali.